Liver

By | 2010-08-29

A solid glandular organ lying in the anterior-most part of the abdomen close up against the diaphragm. Its colour varies from a dark red-brown in the horse to a bluish-purple in the ox and pig; it is soft to the touch though it is rather friable in consistency, and it constitutes the largest gland in the body.

Functions

Functions include the excretion of bile, the storage of glycogen and of iron, the breaking-down of old and worn-out red blood cells, and the breaking-down of toxic substances and of waste substances from the tissues of the body. From the liver, urea and uric acid find their way into the bloodstream and are excreted from the body in the urine by the kidneys. In animals except those of the horse tribe, the bile is collected in the gall-bladder and the bile-duct before passing to the small intestine, where it assists the pancreatic juice in the digestion of food after a meal.

Shape

There are probably few organs which vary so much in shape as the liver, not only in different animals; but also in different individuals of the same species.

Horse

It lies obliquely across the abdominal surface of the diaphragm, its highest and most posterior part being at the level of the right kidney. It possesses a strongly convex diaphragmatic surface which is moulded into the concavity of the diaphragm, and a posterior or abdominal surface which lies in contact with the stomach, duodenum, and right kidney, each of which organs forms a depression in the liver substance. It is only incompletely divided into 3 lobes in the horse. Lying mainly in the right lobe on its abdominal surface is the ‘porta’ of the liver, where the portal vein and hepatic artery enter and from whence the hepatic duct (bile-duct) emerges. Part of the posterior vena cava passes through the liver substance, whose blood it eventually drains. The liver is held in position by the pressure of other organs and by 6 ligaments. These are: the coronary, which attaches it to the diaphragm; the falciform, from the middle lobe to the diaphragm and abdominal floor; the round, to the umbilicus; the right lateral, to the costal part of the diaphragm; the left lateral, to the tendinous part of the diaphragm; and the hepatorenal or caudate, to the right kidney.

Cattle

The liver lies mainly to the right of the middle-line through the body, and its long axis is directed downward and forward. Its diaphragmatic surface fits into the concavity of the right part of the diaphragm, and its posterior surface is very irregular. It presents impressions of the 2 main organs with which it comes into contact — the omasum and reticulum. There is only 1 distinct lobe — the caudate. There is no left lateral ligament, and the round ligament is only found in the calf. A gall-bladder is present; it is situated partly in a slight depression on the posterior surface of the liver, and partly on the abdominal wall.

Sheep

The bile-duct joins the pancreatic to form a common duct instead of opening separately as in other animals.

Pigs

The liver is large, very thick, and very much curved. It lies in the anterior part of the abdominal cavity, occupying the whole of the anterior hollow of the diaphragm and more to the right than to the left side of the body. It has 4 main lobes.

Dogs

The liver is very large, being about 5 per cent of the whole bodyweight, and possesses 6 or 7 lobes. The gall-bladder is buried almost completely in the space between the 2 parts of the right central lobe, only a very small portion of it being visible from the outside.

Minute structure

The liver is enveloped in an outer capsule of fibrous tissue with which is blended the hepatic peritoneum. The hepatic artery, portal vein, and bile-duct divide and subdivide. Between the rows of liver cells also lie fine bile capillaries which collect the bile discharged by the cells and pass it into the bile-ducts lying around the margins of the lobules. The liver cells are amongst the largest cells of the body, and each contains 1 large nucleus. With careful special staining methods there can also be seen tiny passages or canals, passing into the cells themselves; some of these communicate with the bile-duct, and others with the ultimate branches of the portal vein. After a mixed meal many of the liver cells can be seen to contain droplets of fat, and granules of glycogen (animal starch) can also be determined. In addition to the cells above described, there occur at intervals along the walls of the sinusoids in a lobule stellate cells which represent the remains of the endothelium from which the capillary-likesinusoids are developed. They are known as ‘Kupfer’s cells’.